Lesions of bulbo-spinal serotonergic or noradrenergic pathways reduce nociception as measured by the formalin test

Intrathecal administration of the neurotoxins 5,6-dihydroxytryptamine (5,6-DHT) and 6-hydroxydopamine (6-OHDA) in rats selectively lesioned the descending spinal serotonergic and noradrenergic pathways, respectively. Four days after neurotoxin administration the behaviour was evaluated in the formalin test. Several behavioural variables were recorded. The statistical analysis of the results was supplemented using a multivariate statistical method (partial least squares projection to latent variables, PLS) in addition to traditional analysis of variance. The described methods for recording and statistical analysis of behaviour appear to be useful in describing drug-induced differences in behavioural patterns in the formalin test. Both types of lesion reduced the pain-related behaviour in the formalin test (protection of the paw, biting and licking). The results indicate that the descending monoaminergic pathways are parts of a network which maintains adequate nociceptive responses to a chemical stimulus, or to stimuli lasting several minutes, as in the formalin test.